1. Field of the Invention
The present invention relates to an alignment method for aligning optical axes of an optical fiber array and a microlens array which form an optical module, the optical module aligned by the alignment method, and a device for performing the alignment method.
2. Description of the Prior Art
An optical module for optical communication which coupled an optical fiber array to a microlens array is known. This module is provided to cause light from a light emitting diode to enter the microlens through an optical fiber and to take it out as collimated light or to cause the collimated light to enter the optical fiber through the microlens.
In such an optical module, it is essential for the optical axes of the optical fiber and the microlens to agree with each other for improvement of communication accuracy. An optical axis adjustment method for this optical module is disclosed in Japanese Unexamined Patent Publication No. Hei 9-061666.
In this prior art, a mask having a mesh-shaped pattern of the same array pitch as that of an optical fiber array and a collimation lens array (i.e., microlens array) is provided in front of a detector which detects light beam shape. Light is caused to enter the collimation lens array through the optical fiber array, and the light emitted from the collimation lens array and passed without being blocked off by the mask is sensed by the detector. A relative position of the optical fiber array and the collimation lens array is adjusted so that the light beam shape corresponding to each optical fiber can be uniform.
However, even though the optical axis adjustment is made according to the conventional method stated above, it is only possible to make extremely rough adjustment. Further, even when the relative position of the optical fiber array and the collimation lens array is adjusted, it is still not clear in which direction and to which extent the adjustment should be made.
To solve the problems stated above, an alignment method of an optical module according to the present invention is provided, in which alignment of the optical module comprising an optical fiber array in which a plurality of optical fibers are disposed in one or two-dimensional manner and a microlens array in which a plurality of microlenses are disposed in one or two-dimensional manner is made, characterized in that a laser beam emitted from a laser beam source is split into object light and reference light and the object light is caused to enter at least one of the plurality of optical fibers and to emit from the microlens array, wherein the reference light is provided to overlap the emitted object light so as to generate interference patterns, thereby making fine adjustments to relative position of the optical fiber array and the microlens array based on the interference patterns.
An alignment device of the optical module according to the present invention comprises a means for splitting a laser beam from a laser beam source into object light and reference light, a means for guiding the object light to an optical fiber, a light coupling means for overlapping the object light emitted from a microlens array and the reference light each other to generate interference patterns, light observation means for observing the interference patterns, and a means for make fine adjustment to relative position of an optical fiber array and the microlens array based on the interference patterns.
In this manner, the interference patterns of the object light and the reference light greatly change even by slight shift or deviation of the optical axis. Accordingly, it is possible to precisely make fine adjustments to the optical axis using these interference patterns.
An efficient alignment method is provided in which the object light is caused to enter two optical fibers and to overlap reference light so as to generate two interference patterns, wherein fine adjustments are made to relative position of the optical fiber array and microlens array to allow the two interference patterns to approximate. In this case, it is desirable that the two interference patterns be the same with each other, but they don""t have to be completely the same.
To allow the object light to enter the two optical fibers, the object light split by a light splitter is caused to enter the two optical fibers forming the optical fiber array through a coupling member.
For comparison of the two interference patterns, wave front phase analysis software is installed within a control device such as a personal computer, wherein the wave front phase is analyzed from the interference fringes of the two interference patterns to allow these wave front phases to approximate.
It is possible to know the direction and tendency of the optical axis shift or deviation by changing the interference patterns. To change the interference patterns, for example, a phase of the object light or reference light can be shifted. To shift the phase, a phase shifting means is provided in the optical path of the reference light or the optical path of the object light emitted from the microlens array.
For observational interference patterns, the interference patterns on the position spaced away predetermined distance from the microlens array are used. Specifically, the laser beam is a Gausian beam provided with a beam waist. When the beam waist position is set to be an observation position, the wave front phase becomes a plane wave.
The light observation means shall be provided with an image input element such as a CCD camera or a camera tube, and an optical system for forming an image in a position spaced apart a predetermined distance from the microlens on the image input element.
If the alignment is made using the above method and device, the target optical module can be obtained by bonding the optical fiber array and the microlens array together in such a condition.